In photogrammetric aerial cameras, continuously rotating disc shutters are still utilized for generating short exposure times with high lens shutter efficiency since with shutters of this type, the relatively large shutter aperture surfaces needed for this application can be covered rapidly. Rotary disc shutters mostly comprise two disc pairs which are interconnected with a gear transmission and rotate at different speeds. The openings in the discs then come into coincidence periodically with each multiple of both rotation times with the multiple being determined by the transmission ratio. Of the shutter apertures which then are provided, only individual ones thereof are, however, cleared by the actuation of an additional selection disc. A shutter exemplary of this type is disclosed in U.S. Pat. No. 2,926,582.
It is a disadvantage in shutters of this type that the access time t.sub.z is not constant. The access time is the delay time between the actuation of the shutter and the next possible actual exposure time point. Instead of being constant, this access time t.sub.z is dependent upon the speed of the discs or the desired exposure time. This becomes especially noticeable for its undesirable effect for long exposure times, that is, it becomes especially noticeable at low rotational speeds. The access time is dependent upon the instantaneous position in which the disc pairs happen to be and can amount approximately up to a multiple of 200 times the exposure time. For this reason, values of the access time of up to two seconds can result for long exposure times.
However, long exposure times are gaining significance with aerial cameras especially because of the increased use of high resolution films which, however, are less sensitive; this, in turn, is caused by the introduction of devices for compensating for the loss of definition of image movement. It is precisely here that long and non-constant access times are disturbing since precise image overlap values can no longer be obtained.
A great many attempts at a solution are known wherein the attempt was made to shorten the access time in rotating leaf-type shutters or to at least reduce the influence of the overlapping. Accordingly, the suggestion is made in U.S. Pat. No. 4,616,911 that the rotational speed of the rotary discs of the shutter is individually adjusted so that the exposure interval is always a whole-number multiple of the time interval between two possible exposure points of the shutter. Although the fluctuations of the access time are reduced in this manner, long access times result however as previously; that is, this process limits the time of exposure.
For solving the above-mentioned problem, German published patent application 3,514,433 teaches that the magnitude of the sector cutout of the rotary discs can be mechanically changed. However, this can be realized only with relatively large structural complexity.
East German Patent 159,222 discloses a shutter wherein the rotary discs are not coupled by gearing; rather, the rotary discs are individually synchronously rotated via two separate drives with the sector cutouts in the two discs being, however, displaced with respect to each other. To clear the beam path, one disc is accelerated for a short time via an external signal so that the sector cutouts are brought into coincidence with each other. The disc is again reset after the exposure.
Aside from a relatively high complexity on the drive side, precise exposure times can be obtained only with difficulty with this arrangement.